Thin film electroluminescent (TFEL) displays are solid-state flat panel displays available in a variety of colors that encompass a small volume relative to the display surface area. TFEL displays include electronic drive circuitry that creates images in a flat display panel comprising a sandwich of thin film layers opposing a transparent protective faceplate. The sandwich of thin film layers includes front and back electrode layers separated by front and back dielectric layers and a central phosphor layer (luminescent). The drive circuitry creates a luminescent image in the phosphor layer. Light rays, originating in the phosphor layer and projecting from the front surface of the faceplate, allow the image to be seen by a viewer. The display panel is typically formatted as an X-Y matrix of pixels. The electrode layer construction and drive circuitry support the application of individual voltage differences between the two electrode layers at each pixel location. A voltage difference between the electrodes at a particular pixel excites the portion of the phosphor layer within the pixel area, causing the pixel area of the phosphor layer to become luminous. An image is created by the matrix of luminous/nonluminous pixels. The drive circuitry sequentially processes the pixels row-by-row, exciting the appropriate pixels to create the desired image. The luminance of a pixel is proportional to its level and/or its frequency of excitation. As the number of rows of pixels increases, the period of time that can be spent exciting a particular pixel decreases, and therefore the electrical current the drive circuitry applies to the electrodes must increase to achieve the same level of average pixel luminance. Ultimately, the luminance of the display is limited by the current capacity of the drive circuitry, which is related to the ability of the display panel to dissipate heat.
The maximum luminance of presently available TFEL displays is insufficient in certain environments of high ambient light. In addition to being limited by the current capacity of drive circuitry, the luminance of TFEL displays is limited by their low efficiency; the ratio of light energy emitted from the faceplate of a TFEL display to the unit input energy applied to the display's drive circuitry is low, e.g., 1%. Before recent improvements, e.g., development of phosphors with greater luminous efficiency, the efficiency of TFEL displays was even worse. While TFEL displays have improved, a mechanism that creates a significant loss of light energy remains. Specifically, because the dielectric layers adjacent to the phosphor layer of a conventional TFEL display panel have lower indices of refraction than the phosphor layer, light rays originating in the phosphor layer are either reflected at the dielectric/phosphor layer interfaces or pass into the dielectric layers. As a result, a significant portion of the light rays produced are reflected at the dielectric/phosphor layer interfaces and trapped in the phosphor layer, sequentially reflecting between the front dielectric layer/phosphor layer interface and the back dielectric layer/phosphor layer interface. Such light rays are channeled laterally in the phosphor layer and are eventually emitted out a side of the display panel. Thus, they do not contribute to the viewable image.
The just-described mechanism of light energy loss also causes a decrease in contrast. Not all reflected light rays reflect continuously in the phosphor layer until being emitted from a side of the display panel. A significant percentage of light rays that reflect at two or more layer interfaces are emitted from the front surface of the faceplate. Such randomly emitted light rays reduce the contrast of the image produced by the display. Contrast is reduced because these light rays, which are internally channeled laterally from their point of origin in the phosphor layer, are emitted from the surface of the faceplate at a different position and angle than would have occurred if the light rays were not internally reflected. Thus, they appear to have originated from a different position in the phosphor layer. The result is a reduced contrast image.
The present invention is directed to providing a display panel that exhibits a lower percentage of internally reflected light rays and, therefore, provides greater luminance and better contrast than prior art display panels.